Vitamin D, also known as calciferol, is a fat-soluble vitamin essential for bone health, immune function, and overall wellbeing. Unlike most vitamins, your body can produce vitamin D when skin is exposed to sunlight, though many people in the UK require supplementation, particularly during autumn and winter. This article explains what vitamin D calciferol is, who needs supplements, how to recognise deficiency, recommended dosages, potential side effects, and natural sources. Understanding your vitamin D requirements helps protect against conditions such as rickets, osteomalacia, and bone fractures whilst supporting long-term musculoskeletal health.

What Is Vitamin D (Calciferol) and Why Do You Need It?

Vitamin D, also known as calciferol, is a fat-soluble vitamin that plays a crucial role in maintaining bone health, immune function, and overall wellbeing. Unlike most vitamins, your body can synthesise vitamin D when your skin is exposed to ultraviolet B (UVB) radiation from sunlight. The term 'calciferol' encompasses two main forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol), with D3 being the form produced naturally in human skin and generally considered more effective at raising blood levels. For treatment of deficiency, colecalciferol (D3) is typically preferred in the UK.

Once synthesised or consumed, vitamin D undergoes two hydroxylation steps—first in the liver to form 25-hydroxyvitamin D (the storage form measured in blood tests), then in the kidneys to produce the active hormone 1,25-dihydroxyvitamin D. This active form regulates calcium and phosphate absorption in the intestines, which is essential for maintaining adequate mineralisation of bones and teeth. Without sufficient vitamin D, bones can become thin, brittle, or misshapen, leading to conditions such as rickets in children and osteomalacia in adults.

Beyond skeletal health, vitamin D receptors are found throughout the body, including in immune cells, muscle tissue, and the cardiovascular system. Research suggests vitamin D may influence immune regulation, muscle function, and cellular growth, though the full extent of these non-skeletal effects continues to be investigated. The UK's Scientific Advisory Committee on Nutrition (SACN) recommends that everyone aged one year and above should have a daily intake of 10 micrograms (400 IU) of vitamin D to protect musculoskeletal health, particularly during autumn and winter months when sunlight exposure is insufficient for adequate synthesis.

Signs and Symptoms of Vitamin D Deficiency

Vitamin D deficiency can develop gradually, and many people remain asymptomatic until levels become severely depleted. When symptoms do occur, they often relate to the vitamin's role in calcium metabolism and bone health. Common manifestations include:

• Bone pain and tenderness, particularly in the ribs, hips, pelvis, and lower limbs

• Muscle weakness and aches (myalgia), which may affect mobility and increase fall risk

• Fatigue and general malaise that doesn't improve with rest

• Mood changes, including low mood, though the relationship between vitamin D and mental health requires further research

In children, severe deficiency causes rickets, characterised by delayed growth, bone deformities (such as bowed legs or thickened wrists and ankles), dental problems, and skeletal pain. In adults, prolonged deficiency leads to osteomalacia, where bones become soft and prone to fractures, causing persistent bone pain and proximal muscle weakness that may result in a waddling gait.

Subtle signs may be easily overlooked or attributed to other causes. Some individuals report frequent infections, though the evidence linking vitamin D deficiency directly to infection risk is mixed, and supplementation is not routinely recommended solely for infection prevention.

If you experience persistent bone pain, unexplained muscle weakness, or recurrent fractures, you should contact your GP for assessment. Routine population testing is not recommended, but those with symptoms or in high-risk groups should be assessed. Investigations typically include measuring serum 25-hydroxyvitamin D levels (not 1,25-dihydroxyvitamin D) and a bone profile (calcium, phosphate, alkaline phosphatase). Levels below 25 nmol/L are generally considered deficient and levels between 25–50 nmol/L indicating insufficiency. Children with signs of rickets or adults with unexplained fractures or persistent bone pain may need referral to secondary care.

Who Should Take Vitamin D Supplements in the UK?

The Office for Health Improvement and Disparities (OHID) and NHS recommend that certain population groups should take daily vitamin D supplements throughout the year due to increased risk of deficiency. The following groups should consider year-round supplementation:

• Infants and young children aged one to four years should receive a daily supplement containing 10 micrograms (400 IU). Breastfed babies should receive 8.5–10 micrograms daily, while formula-fed infants consuming more than 500 mL of infant formula daily do not need additional vitamin D as formula is already fortified

• People with limited sun exposure, including those who are housebound, in care homes, or who cover their skin for cultural or religious reasons

• Individuals with darker skin (African, African-Caribbean, or South Asian backgrounds), as higher melanin levels reduce vitamin D synthesis

• Pregnant and breastfeeding women, to support foetal skeletal development and maintain maternal bone health. Pregnant women should avoid liver and cod liver oil supplements due to their high vitamin A content

Additionally, all adults and children over four years should consider taking a daily supplement containing 10 micrograms of vitamin D during autumn and winter (October to early March in the UK), when sunlight is insufficient for adequate skin synthesis. This recommendation applies to the general population, as dietary sources alone rarely provide sufficient vitamin D.

Certain medical conditions increase deficiency risk and may warrant higher therapeutic doses under medical supervision. These include malabsorption disorders (coeliac disease, Crohn's disease, chronic pancreatitis), chronic kidney disease, liver disease, obesity, and use of medications that interfere with vitamin D metabolism (such as anticonvulsants, glucocorticoids, cholestyramine, rifampicin, and orlistat). People who have had bariatric surgery should also discuss supplementation with their healthcare provider. If you fall into any high-risk category, consult your GP or pharmacist to determine the appropriate supplementation regimen, as therapeutic doses may differ significantly from standard prophylactic recommendations.

How to Take Calciferol: Dosage and Forms Available

Vitamin D supplements are available in several formulations, with dosing regimens varying according to whether they're used for prevention (prophylaxis) or treatment of deficiency. Standard prophylactic doses for the general UK population are 10 micrograms (400 IU) daily for adults and children over one year. For infants under one year, the recommended dose is 8.5–10 micrograms daily, available as drops specifically formulated for babies. Formula-fed infants consuming more than 500 mL of infant formula daily do not need additional vitamin D supplements.

Calciferol supplements come in various forms:

• Tablets and capsules (vitamin D2 or D3), typically containing 400 IU to 4,000 IU per dose

• Oral drops or liquid preparations, particularly suitable for infants, children, and those with swallowing difficulties

• Chewable tablets, offering an alternative for people who prefer not to swallow capsules

• High-dose preparations (20,000 IU to 50,000 IU), reserved for treating confirmed deficiency under medical supervision

For treatment of deficiency, UK guidelines recommend loading doses followed by maintenance therapy. Typical loading regimens include colecalciferol 50,000 IU weekly for 6 weeks (total 300,000 IU), or 20,000 IU twice weekly for 7 weeks, or 4,000 IU daily for approximately 10 weeks, followed by maintenance of 800–2,000 IU daily. Some patients may require up to 4,000 IU daily maintenance under clinical supervision. The exact protocol depends on deficiency severity, patient factors, and local guidelines.

Vitamin D is contraindicated in hypercalcaemia and metastatic calcification, and should be used with caution in conditions such as primary hyperparathyroidism, granulomatous disorders, or severe kidney disease.

Vitamin D is fat-soluble, so absorption is enhanced when taken with food containing some fat. Take your supplement with a meal to optimise absorption. Consistency matters more than timing—choose a time you'll remember, whether morning or evening. If you're prescribed high-dose therapy, adherence to the schedule is important, but if you miss a dose, simply take the next one as planned rather than doubling up. Always follow the dosage instructions on the product label or as directed by your healthcare professional, and store supplements in a cool, dry place away from direct sunlight to maintain potency.

If you're receiving treatment for deficiency, your doctor may check your adjusted serum calcium about one month after completing loading doses and may recheck your vitamin D levels after 3–6 months to confirm response.

Potential Side Effects and Safety Considerations

When taken at recommended doses, vitamin D supplements are generally well-tolerated with minimal adverse effects. However, excessive intake can lead to vitamin D toxicity (hypervitaminosis D), which causes hypercalcaemia—elevated calcium levels in the blood. This condition develops gradually with prolonged excessive supplementation, typically from doses exceeding 4,000 IU daily over extended periods, though individual tolerance varies.

Symptoms of vitamin D toxicity include:

• Nausea, vomiting, and loss of appetite

• Excessive thirst and frequent urination

• Constipation and abdominal pain

• Weakness, confusion, and fatigue

• Kidney problems, including kidney stones and, in severe cases, renal impairment

The UK's Expert Group on Vitamins and Minerals has set a safe upper level for long-term daily intake at 100 micrograms (4,000 IU) for adults. It is virtually impossible to achieve toxic levels through sunlight exposure or diet alone—toxicity results from excessive supplementation. If you're taking multiple supplements or fortified foods, check total vitamin D content to avoid unintentional over-supplementation.

Certain individuals require careful monitoring when taking vitamin D. People with sarcoidosis, tuberculosis, lymphoma, or primary hyperparathyroidism may be more susceptible to hypercalcaemia and should only take supplements under specialist supervision. Those with kidney stones or chronic kidney disease should discuss supplementation with their doctor, as vitamin D affects calcium metabolism.

Vitamin D can interact with several medications, including thiazide diuretics (increasing hypercalcaemia risk), digoxin (where high calcium levels increase toxicity risk), cholestyramine (reducing absorption), orlistat (reducing fat-soluble vitamin absorption), rifampicin (increasing vitamin D metabolism), and glucocorticoids (affecting vitamin D metabolism).

Contact your GP immediately if you experience persistent nausea, excessive thirst, confusion, or other concerning symptoms whilst taking vitamin D supplements. Seek urgent assessment if you develop severe symptoms of hypercalcaemia such as confusion, dehydration, or irregular heartbeat. Regular monitoring of serum calcium and vitamin D levels may be recommended for those on high-dose therapy or with underlying health conditions.

Report any suspected side effects from vitamin D supplements to the MHRA Yellow Card Scheme.

Food Sources and Sunlight: Natural Ways to Boost Vitamin D

Whilst supplementation is often necessary in the UK, optimising natural vitamin D sources through sensible sun exposure and dietary choices provides additional support for maintaining adequate levels. Sunlight remains the most efficient natural source—when UVB rays strike the skin, they convert 7-dehydrocholesterol to previtamin D3, which then isomerises to vitamin D3. In the UK, adequate synthesis only occurs between late March and September, when the sun is at a sufficient angle.

During these months, short periods of sun exposure without burning can support vitamin D production. The amount of time needed varies with skin type, time of day, and season. Those with darker skin may require longer exposure. However, balancing vitamin D synthesis with skin cancer prevention is important. Never allow your skin to redden or burn, and follow NHS sun safety guidance, particularly during peak UV hours. For most people in the UK, particularly during autumn and winter, sunlight exposure alone cannot maintain adequate vitamin D levels, making dietary sources and supplementation essential.

Dietary sources of vitamin D include:

• Oily fish (salmon, mackerel, sardines, herring)—the richest natural food source, providing approximately 7–25 micrograms per 100g

• Egg yolks—containing approximately 1–2 micrograms per egg

• Fortified foods, including breakfast cereals, fat spreads, and some plant-based milk alternatives

• Red meat and liver—providing small amounts (approximately 0.5–1 microgram per serving). Pregnant women should avoid liver and cod liver oil due to high vitamin A content

• Mushrooms exposed to UV light—containing variable amounts of vitamin D2

In the UK, infant formula milk is fortified with vitamin D, which is why formula-fed babies consuming more than 500 mL daily do not require supplementation. Breast milk contains insufficient amounts, which is why breastfed babies require vitamin D drops.

Few foods naturally contain significant vitamin D, and achieving the recommended 10 micrograms daily through diet alone is challenging without regular consumption of oily fish or fortified products. A balanced approach combining sensible sun exposure during summer months, incorporating vitamin D-rich foods into your diet, and taking supplements during autumn and winter (or year-round if in a high-risk group) provides the most reliable strategy for maintaining optimal vitamin D status and supporting long-term bone and overall health.

Frequently Asked Questions